Prototropic tautomerism in N,N-dimethyl-N′-(1-nitro-9-acridyl)propane-1,3-diamine and its nitro isomers. Application of MNDO and PPP methods for the examination of structure and electronic absorption spectra

Abstract

N,N-Dimethyl-N′-(1-nitro-9-acridyl)propane-1,3-diamine (Ledakrin® or nitracrine), which exhibits distinctive anticancer activity, and its three nitro isomers can potentially exist in two or three tautomeric forms (amino, imino, aci). This is due to the possibility of migration of the hydrogen atom which is bound to the nitrogen atom attached to carbon C-9 to the acridine ring nitrogen, or to the nitro group. The structures of several possible tautomers were determined by geometry optimization using the MNDO method. The latter procedure enabled also the evaluation of (i) the heats of formation, (ii) the dipole moments, and (iii) the ionization potentials to be undertaken. From these structures the frequencies of electronic transitions and the oscillator strengths for individual tautomeric forms were calculated using an adaptation of the PPP method. The evaluated UV absorption spectra were compared with experimental data. It was revealed that the observed absorption in the long-wavelength region is characteristic for a given molecule. Moreover, in the case of a given isomer the absorption can be readily explained by the results of theoretical calculations presented here, assuming that the molecule may occur in various accessible tautomeric forms.